“grind for another.” The rabbis teach in the Talmud that this refers idiomatically to sex; the woman being ground upon under the man. That would make sense because the context is the retribution Job would receive for having committed adultery with another man’s wife (Job 31:9) and because it fits with the last stanza in the verse, which is clearly about another man having sex with Job’s wife. However, it is also possible that the verse refers to the normal practice of the woman of the house grinding grain, but for Job’s wife to do that in another man’s house would require Job to suffer great misfortune or death.
The kind of grinding that would most fit the sexual context in this verse is where there is a flat stone on the bottom on which the grain is placed, and the person grinding uses a cylindrical stone with narrow ends (much like a rolling pin is shaped) and rubs the grinding stone back and forth over the grain to grind it.
[For more on grinding grain, see commentary on Deut. 24:6.]